(1) Field of the Invention
The present invention relates to an image forming apparatus, and especially relates to an image forming apparatus provided with a photosensitive drum which is driven by a stepping motor.
(2) Related Art
In an electrophotographic type image forming apparatus, a toner image is formed on a photosensitive drum and the toner image is transferred onto a recording sheet to form an image. To transfer the toner image formed on the photosensitive drum onto the recording sheet with fidelity, a circumferential speed of the photosensitive drum needs to be precisely equal to a transportation speed of the recording sheet. For this reason, in recent years, a stepping motor is used as a driving source of the photosensitive drum for its high performance in braking in comparison with other kinds of motors.
As one example of conventional image forming apparatuses, Japanese Laid-Open Patent Application No. 4-34444 discloses an image forming apparatus which includes a photosensitive drum driven by a stepping motor whose rotational speed is decreased to 1/129 via a reducer composed of pulleys and a belt. Using this conventional image forming apparatus, one scanning line of the electrostatic latent image is formed on the surface of the photosensitive drum every time the stepping motor rotates four steps.
However, with the construction of the conventional image forming apparatus, rotation of the stepping motor is not reliably transmitted to the photosensitive drum due to problems such as the belt skidding on the pulleys or expansion and contraction of the belt. This leads to nonuniformity in rotation of the photosensitive drum and in the distances between the scanning lines, and so causes inconsistency in the print density on the reproduced image.
To address this problem, the photosensitive drum can be directly driven by a stepping motor. However, using the cited image forming apparatus, about 32 scanning lines (obtained by calculating an equation 129/4) of the electrostatic latent image are formed every time the stepping motor rotates one step. As known in general, the rotational speed of the stepping motor during one step is not uniform. Consequently, the distances between the 32 lines are not uniform, and so causes inconsistency in the print density on a reproduced image.